Ick on a Mandarin?

[hillegom]

If its ich, if one fish has it, all of them could be infected. Some just can deal with ich better. Ich usually infects the gills first, so its hard to spot.
If you cure the mandarin, out of the tank, when he goes back in, he could be infected again.
I would treat all the fish, and leave your DT fallow for 8 weeks